1. Field of the Invention
This invention relates to a chip type high frequency coil device including electrodes adapted for surface-connection with a circuit pattern provided on a substrate.
2. Description of the Prior Art
Recently, with the miniaturization of electronic components, so-called chip-type high frequency coil devices have been proposed which are provided with electrodes adapted for direct surface-connection with a circuit pattern formed on a substrate formed of a plastic or ceramic material. Generally, such coil devices are so small as to be contained in a cube each side of which is as short as about 4 to 5 mm, and thus they are attended with technical difficulties which have not been encountered with the conventional high frequency coil devices including a base member provided with terminal pins. Among such technical difficulties is one which is experienced in an attempt to securely attach the shield casing of the coil device.
Referring to FIG. 1, there is shown, in a side view, a shield casing for a conventional high frequency coil device including terminal pins. FIG. 2 is a schematic longitudinal sectional view of the conventional high frequency coil device. As will be seen from these figures, the shield casing 1 is fitted over a base member 4 formed of a plastic material, and provided with projections 3 which are formed by inwardly stamping opposite side walls 2 of the casing. The shield casing 1 is fixed to the base member 4 by disposing the projections 3 in engagement with the upper surface of the base member 4 having terminal pins planted therein, and by bending lugs into contact with the bottom surface of the base member 4 so that the latter is held between the projections 3 and the lugs 5. Indicated at 6 is a threaded portion with which is engaged a cap core 8 which is disposed in such a manner as to cover a winding portion 7 formed by winding a coil on a drum core placed on the base member 4. Denoted at 9 are lugs adapted to serve as ground electrodes.
However, difficulties are experienced in an attempt to employ the above-mentioned fixing procedure with respect to a chip type high frequency coil device which is miniaturized as mentioned above.
More specifically, in the case of a chip type high frequency coil device, since it is too small in size, no base member is provided, and the lower flange of a drum core formed of ferrite is arranged to serve as such a base member; thus, it is required that a shield casing be secured directly to the lower flange which is small and slippery. Moreover, the shield casing per se is so small that projections and lugs provided thereon are correspondingly small, and hence the shield casing cannot be completely fixed simply by holding the lower flange of the drum core between the projections and the lugs. Obviously, this may permit the shield casing to be displaced, and thus aggravate the drawback inherent with the chip type high frequency coil device that the characteristics thereof tend to become unstable due to the fact that the device is poor in respect of working accuracy and physical strength since it is too small. Therefore, it is the status quo that chip type high frequency coil devices wherein the inductance thereof is adjustable through the engagement of the cap core with the shield casing, have not yet predominantly been put to practical use.